Combustion devices based on hydrocarbon fuels are widely used to provide thermal, mechanical or electric energies. For example, fireplaces, ovens, furnaces, and boilers have been installed and used in commercial and residential buildings to provide heat, hot water, and other conveniences. Ideally, complete combustion occurs when hydrocarbon compounds in the fuel exothermically react with oxygen in the air to produce water vapor and carbon dioxide. Furnace systems are designed to run the combustion reaction with an excess of oxygen so that complete combustion can take place and maximum amount of heat may be released from hydrocarbon fuels.
A conventional condensing furnace system for a residential building typically includes a burner operatively connected to a heat exchanger, a combustion air intake pipe operatively connected to the burner, and an exhaust pipe operatively connected to the heat exchanger by way of a draft inducer. In use, ambient air from outside of the building is induced into the furnace system through the intake pipe that extends through a building wall. The induced intake air is then fed into the burner, where the hydrocarbon fuel is injected and entrenched in the induced intake air. The fuel-air mixture is then combusted to produce a flame that flows into the heat exchanger, where the heat generated from the combustion is transferred to another medium (air or water to be heated). The exhaust gas (flue gas) is then discharged from the heat exchanger to outside of the building through the exhaust pipe, also extending through a building wall.
The intake and exhaust pipes may be integrated into a compact tube-within-tube design for easier installation and/or cost and space saving. For example, the exhaust pipe may be concentrically disposed within the intake pipe. As a result, while flue gas is discharged through the exhaust pipe, ambient air is induced into the furnace system through the annular space between the intake and exhaust pipes. As the intake and exhaust pipes are generally made of Polyvinyl Chloride (PVC) or other gas impermeable material, no substance is transferred between the intake air and flue gas.
On the other hand, the intake and exhaust pipes may also have a side-by-side configuration to improve the efficiency of the furnace by promoting heat exchange between the intake air and flue gas, i.e. pre-heating of the intake air by the flue gas. To that end, a membrane module may be disposed between the intake and exhaust pipes to promote heat exchange therebetween. The membrane module may also simultaneously allow moisture exchange between the intake air and flue gas. The moisture exchange may also reduce NOx emission of the furnace.
However, the construction of the membrane module is relatively complicated and requires, for example, an array of parallel exhaust tubes made of a hydrophobic polymeric material and orthogonally disposed in the flow path of the intake air. Accordingly, the membrane only extends along a small section of the intake and exhaust pipes. As a result, the heat and/or moisture exchange capacities of the membrane module are limited. Moreover, the membrane module requires circulation of a moisture absorbent, such as a hygroscopic liquid like ethylene glycol, which not only increases manufacturing and maintenance costs of the furnace system but may also cause undesirable noises as the flowing intake air and/or flue gas interacts with the hygroscopic liquid.
Hence, there is a need for a vent for a combustion device that combines the intake and exhaust pipes into a compact and easy to install apparatus while improving the efficiency of the combustion device and/or reducing emission of same. Further, there is a need for a furnace vent with simple construction and low maintenance (i.e. no complex membrane module design or hygroscopic agent).